This invention relates to a method and apparatus designed to automatically calibrate a spectrophotometer. Spectrophotometers are optical instruments employed for both the qualitative and quantitative analysis of a variety of materials. Samples having unknown constituents are analyzed by sequentially illuminating a sample with narrow wavelength bands across the spectrum of an elected range of electromagnetic radiation and then measuring the intensity of light transmitted or reflected from the sample. Light can be dispersed into its constituent band wavelengths by a grating or a conventional prism. In spectrophotometers, gratings are employed because of their ability to disperse light into narrow wavelength bands so that the light in the bands is close to being monochromatic. The grating is oscillated with respect to the light source so that light reflected from the grating passes through a narrow slit. The light exiting the slit is restricted to a narrow wavelength band which is scanned through a wavelength range of the desired region of the spectrum. Light passing through the exit slit is directed to a sample to be analyzed and photodetectors measure the light either reflected or transmitted by the sample. Near infrared spectrophotometry, involving the use of electromagnetic radiation of wavelengths between 1100 Angstrom and 2500 Angstrom, is a particularly effective tool in the analysis of organic compounds because of their significant exhibition of absorptive characteristics in the near infrared region of the spectrum.
Although the spectrophotometers as described above are effective at measuring absorptive properties of materials, it has been difficult to maintain precise calibration of the instruments to ensure accuracy of the absorbance measurements. These problems are manifested as variations in the precise wavelength that pass through the exit slit at a given angular position of the grating with respect to the light source. The center wavelength passing through the exit is related to the angular position by a trigonometric expression which has constants that are unique to each instrument. However, the constants are subject to variations with temperature and drift with time. As a result, the constants of the instrument can change with each measurement and measurements taken with the instruments lose precision and are not precisely repeatable.
In order to compensate for the variations in the instruments, the spectrophotometers are routinely calibrated. In the past, one method of calibration was performed by inserting a standard, such as polystyrene, into the path of light exiting the housing of the spectrophotometer which contained the grating. The standard selected had peaks of absorbance occurring at precisely known wavelengths. By determining the angular positions of the grating where the known peaks occur, the coefficients for the instruments could be calculated and then entered into the instrument's computer. Coefficients entered into the computer would remain unchanged until it was determined that the instrument should be recalibrated. Often the determination that an instrument should be recalibrated was not made frequently enough to maintain the precision of the measurements up to the instrument capability. The present invention increases the precision and reliability of each instrument by automatically recalibrating the instrument before each measurement of an unknown sample in a manner invisible to the user. Thus, the present invention provides an apparatus providing more accurate measurements.